Spatial-temporal Evolution Characteristics And Simulation Trends Of Urban Expansion In Chengdu-Chongqing Twin-City Economic Circle

The increasingly dramatic changes in urban construction land during urbanization are important factors affecting regional and even global socio-economic development and ecological stability.How to grasp the spatial and temporal evolution characteristics of urban expansion and simulate the spatial dynamic pattern of future cities has become an important research content for cities seeking sustainable development.The research on the spatial and temporal variation characteristics and future development direction of urban construction land in the Chengdu-Chongqing Economic Circle has certain theoretical and practical guidance for optimizing the layout of urban construction land in the Chengdu-Chongqing Economic Circle and promoting the development of the western region.In this paper,urban construction land data of Chengdu-Chongqing region extracted from Landsat series remote sensing images and its derived data with a 3-year cycle from 1990 to 2019.With the help of Arc GIS10.2,Fragstate4.2,Origin2021 b,Excel and other software and urban expansion index,urban expansion difference index,circle gradient analysis and standard deviation ellipse analysis,the dynamic evolutionary characteristics of urban expansion of each city in the twin-city economic circle of Chengdu-Chongqing region are studied in depth and the city types are classified.Meanwhile,this study uses the U-Net model to forecast and simulate the spatial distribution of urban land in the twin-city economic circle of Chengdu-Chongqing region in 2035.This study provides a reliable scientific basis for further revealing the spatial and temporal evolutionary characteristics of urban construction land expansion and the inherent differences of landscape changes in typical western urban agglomerations,as well as for the layout and regulation of urban construction land in the Chengdu-Chongqing Economic Circle.the results show that:(1)With the GEE platform,we have effectively acquired long time series,high precision and large scale urban land data of the twin-city economic circle of Chengdu and Chongqing.The overall classification accuracy of urban construction land in the study area is higher than 94 %.The obtained urban land use data lay a solid foundation for the subsequent study of the spatial and temporal characteristics of urban expansion in the Chengdu-Chongqing Economic Circle,as well as for the simulation of urban land use in the economic circle in 2035 and the analysis of its prediction results.(2)The spatial and temporal expansion characteristics of urban land of different scales are compared and analyzed by using the methods of urban land expansion intensity difference index,circle gradient analysis method and urban land density function.The results show that:(1)The urban expansion of the study area has obvious spatial differences,with the twin cores of Chengdu and Chongqing expanding at a faster rate,driving the development of cities along and around them,but overall the development of small and medium-sized cities is relatively lagging behind,and the structure of the urban system and regional synergy needs to be strengthened.(2)There are significant differences in urban land expansion among cities in the study area,showing an overall trend of unbalanced layout.The high-speed expansion areas are mainly distributed in the eastern region.The intensity of urban expansion in the east is higher than that in the west,and the center of rapid expansion of the overall economic circle moves from east to west.(3)In terms of city compactness(Kp)and expansion rate(D),the radius of cities expands over time,and the compactness of most large and medium-sized cities decreases,while small cities tend to be compact.(3)The spatial distribution pattern of urban construction land in the twin-city economic circle of Chengdu-Chongqing region in 2035 is simulated based on the U-Net model,and the results show that:(1)Using the U-Net deep learning model to predict urban land expansion captures linear features well and is very effective in learning urban morphological features.In general,large cities have a much higher potential for expansion than surrounding small and medium-sized cities and the projected urban areas follow a specific pattern rather than expanding in all directions.(2)Landscape expansion index reflects the dynamic change of urban construction land expansion mode.From 1990 to 2035,the enclave expansion mode of the Chengdu-Chongqing Twin-City Economic Circle decreased,the filling and marginal modes increased,and the regional urban construction land was more concentrated.(3)The landscape pattern index showed that during the study period(1990-2035),the urban land use in the Chengdu-Chongqing Economic Circle showed a trend of increased landscape fragmentation,enhanced landscape heterogeneity,and more complex patch shapes.